Demembranated sperm flagella from sea urchin and bull are reactivated with ATP in order to study the properties of their contractile apparatus. The changes in properties caused by extraction with 0.5 M KCl, and by brief digestion with trypsin, provide information concerning the role of the dynein arms in causing active sliding movements between the flagellar tubules. The several dynein fractions in the flagella have been separated by chromatography, and their individual properties are being studied. High-voltage electron microscopy is being used to study the structure of the radial spokes, nexin links, and other structural components in bent regions of the flagella, in order to obtain information concerning the possible roles of these structures in coordinating the sliding movements to produce propagated bending waves.